ARTICLE pubs.acs.org/crystal
Epitaxial Growth of High Quality Ge Films on Si(001) Substrates by Nanocontact Epitaxy Yoshiaki Nakamura,*,†,‡ Akiyuki Murayama,§ and Masakazu Ichikawa§ †
Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan PRESTO, JST, 4-1-8 Honcho Kawaguchi, Saitama 332-0012, Japan § Department of Applied Physics, Graduate School of Engineering, The University of Tokyo, Bunkyo-ku, Tokyo 113-8656, Japan ‡
ABSTRACT: We have developed a novel epitaxial growth technique called nanocontact epitaxy that allows high quality Ge films to be epitaxially grown on Si(001) substrates using spherical nanodots as seed crystals. The nanodots only make contact with the Si substrate through nanowindows in an intermediate ultrathin SiO2 film, and they were elastically strain-relaxed without misfit dislocations. Ge films with a thickness of 130 nm were epitaxially grown on the nanodot seed crystals by solid source molecular beam epitaxy. The resulting Ge films typically exhibited a surface roughness of ∼0.4 nm and an etch pit density of 104 to 105 cm 2, demonstrating their high crystallinity. The films exhibit photoluminescence around 0.8 eV related to Ge crystals.
I. INTRODUCTION Epitaxial growth of Ge films on Si substrates has been studied intensively because the properties of these films are promising for various kinds of applications, such as light emission in optoelectronic integrated circuits, a channel material in field-effect transistors, a material in high-speed optoelectronic devices (transistor or photodetector),1,2 and virtual substrates for GaAs films.3,4 In general, heteroepitaxial growth of high quality films is difficult in systems with large lattice mismatch. In the case of epitaxial growth of Ge/Si with a lattice mismatch of ∼4%, some strain is elastically relaxed by degrading the surface flatness, namely Stranski Krastanov growth,5 while some is relaxed by introducing misfit dislocations. To solve this problem, a number of epitaxial growth techniques have been used to produce Ge/Si films, such as low-temperature molecular beam epitaxy (MBE),6 depositionannealing cycles,7 and epitaxial lateral overgrowth (ELO).8 11 Despite much effort, the heteroepitaxial growth of Ge films remains problematic in terms of surface flatness and crystallinity. We have developed a technique for growing ultrahigh density (>1 1012 cm 2) nanodots (NDs) epitaxially on Si substrates using ultrathin SiO2 films.12 14 As a typical example of this ultrathin SiO2 film technique, a scanning tunneling microscope (STM) image and reflection high energy electron diffraction (RHEED) pattern of the resulting Ge NDs are shown in Figure 1a and its inset, respectively. These NDs were grown epitaxially on the Si substrate by only allowing limited contact between the NDs the Si substrate through nanowindows in the ultrathin SiO2 film. Such nanocontacts result in a small strain energy in the NDs, and the NDs are elastically strain-relaxed and also lack misfit dislocations because of their spherical shape.15,16 A typical example of the Ge NDs is shown in a cross-sectional r 2011 American Chemical Society
high-resolution transmission electron microscope (HRTEM) image in Figure 1b, exhibiting the difference in the spacing of the (111) planes between Ge NDs and the Si substrate (∼4%). The elastic strain relaxation in the Ge NDs was confirmed by the difference in lattice spacing being close to the Si Ge lattice mismatch (∼4%) and the absence of misfit dislocations near interfaces. Recently, we proposed the idea of heteroepitaxial growth using elastically strain-relaxed NDs without misfit dislocations as seed crystals,17 which we have named nanocontact epitaxy. The elastic-strain relaxation means the extinction of the driving force of misfit dislocation formation. This method of growth is quite different from ELO using thin SiO2 films.8 11 In this study, we have expanded this idea to develop a method that allows epitaxial growth of high quality Ge films on Si substrates using Ge ND seed crystals as templates. Typically, the resulting Ge films possess a surface roughness of ∼0.4 nm and an etch pit density of 104 to 105 cm 2. The Ge films exhibit photoluminescence around 0.8 eV related to the Ge crystals.
II. EXPERIMENTAL SECTION Nanocontact epitaxy is composed of three stages, as shown in Figure 1c. Ultrahigh density NDs are epitaxially grown on Si substrates as an initial layer using our previously described ultrathin SiO2 film technique.12,13,15 NDs fabricated by this method are elastically strainrelaxed with lattice constants that are almost the same as that of the bulk material. A second layer is grown to flatten the growth surface, and then a Received: May 13, 2011 Revised: June 3, 2011 Published: June 07, 2011 3301
dx.doi.org/10.1021/cg200609u | Cryst. Growth Des. 2011, 11, 3301–3305
Crystal Growth & Design
ARTICLE
Figure 1. (a) STM image and RHEED pattern (inset) of Ge NDs formed by deposition of 6 MLs of Ge at 600 C. (b) Cross-sectional HRTEM image of Ge NDs formed by deposition of 5 MLs of Ge at 500 C. The two arrows indicate the 10 bilayers of (111) planes in the Ge NDs and Si substrate. (c) Schematic diagram of the proposed epitaxial growth method of Ge films.
Table 1. Conditions Used for Sample Formation sample no.
initial layer of NDs
second layer
final layer
1
36 ML, 540 C
399 ML, 300 C
435 ML, 500 C
2 3
36 ML, 540 C 36 ML, 540 C
399 ML, 200 C 399 ML, 150 C
435 ML, 500 C 435 ML, 500 C
4
9 ML, 540 C
426 ML, 200 C
435 ML, 500 C
5
4 ML, 540 C
431 ML, 200 C
435 ML, 500 C
final layer is grown to obtain a high crystalline flat film. Ge was grown on Si substrates by solid source MBE as follows. Chemically cleaned p-type Si(001) substrates were introduced into a MBE chamber equipped with a RHEED at a base pressure of ∼1 10 8 Pa. Clean Si surfaces were prepared by epitaxial growth of Si buffer layers with a thickness of 100 nm on Si(001) substrates at 500 C after degassing at 500 C for several hours. The clean Si surfaces were then oxidized at 500 C for 10 min at an oxygen pressure of 2 10 4 Pa to form ultrathin ( 500 C, 3-dimensional (3-D) island growth occurred on the NDs. Therefore, growth at TS < 300 C was investigated in detail. Figure 2b d shows RHEED patterns of the second layers, which strongly depended on TS. In the case of a relatively high TS (200 300 C), streaky diffraction patterns were observed (Figure 2b and c). At TS = 300 C (Figure 2b), small spotty diffraction patterns (white arrow) were found in addition to the streaky diffraction patterns. The final layers of Ge were grown at 500 C on the second layers deposited at TS = 300 and 200 C (samples 1 and 2, respectively). The RHEED patterns of samples 1 and 2 are displayed in parts e and f, respectively, of Figure 2. Streaky diffraction patterns with low background were observed for both samples, indicating flat surfaces and high crystallinity. Closer comparison of these RHEED patterns reveals relatively spotty diffraction patterns in sample 1, as highlighted by the arrow in Figure 2e. This shows that sample 2 has a flatter surface than sample 1. SEM images (Figure 2g and h) show that the surfaces of samples 1 and 2 are composed of flat regions and pits 3302
dx.doi.org/10.1021/cg200609u |Cryst. Growth Des. 2011, 11, 3301–3305
Crystal Growth & Design
ARTICLE
Figure 4. SEM image of the surface of samples (a) 4 and (b) 5. (c) AFM image of sample 4.
Figure 3. RHEED patterns of initial layers of NDs containing (a) 4 and (b) 9 MLs of Ge. RHEED patterns of second layers grown at 200 C on initial layers composed of (c) 4 and (d) 9 MLs of Ge. RHEED patterns of final layers of samples (e) 4 and (f) 5.
of submicrometer size (200 300 nm); the pit densities in samples 1 and 2 are 1 108 and 1 107 cm 2, respectively. Thus, sample 2 has a flatter surface than sample 1, which is consistent with RHEED results. In the case of relatively low TS (
